Technical Abstract:
Progress toward improving cow efficiency has been slow. Numerous factors have contributed toward the lack of progress. The first is having a common definition of cow efficiency. Efficiency can be measured at different levels of aggregation in the production system. We can measure the economic efficiency of a cow, and we can measure the biological efficiency. Frequently, maximum output is not associated with maximum efficiency. Determining the desired level of efficiency is partially based on determining the desired level of output. In a given production system, a decrease in the biological efficiency may be offset by an increased economic value of the output that increases the economic efficiency. Selecting the tools to use to improve cow efficiency will greatly depend on an individual’s definition of efficiency. Factors affecting the biological efficiency of the cow can be put into two general categories: 1) productive life, and 2) feed efficiency. The cow’s nutrient requirements change throughout the year and the level of nutrients required at any given time is dependent on the stage of the production cycle. Energy usage by a mature cow can be divided into four general categories: energy required for activity, maintenance, pregnancy, and milk. Energy used for activity is the most variable. Grazing behaviors, terrain, forage availability and water availability all contribute to the variability in activity energy expenditure. Maintenance is the amount of energy required by a cow for no change in energy gain or loss. Cow size and metabolic rate are the two principal factors affecting maintenance requirements. The energy required for fetal growth increases as gestation increases, and the greatest energy requirement is during the last third of gestation. Energy required for milk has two fates. The energy is either deposited in milk or it is lost as heat. Daily maintenance requirement is primarily a function of body weight. The initial reaction to reduce maintenance cost in the cow herd is to reduce the size and or milk production of the cow. Some EPDs for “cow maintenance” have been developed using these traits. Reducing mature cow size will typically also reduce calf body weight at any given age resulting in fewer pounds to market at a given age. Matching the amount of milk that cows need to produce to raise a calf is an important factor in selecting cows for a production system. There has been considerable interest in improving feed efficiency in beef production. There are numerous measures of feed efficiency including gain:feed (G:F) ratios and residual feed intake (RFI) that have been used as an index. It is important to remember that both G:F and RFI are composite traits that fail to describe biological mechanisms associated with increased feed efficiency. Use of these measures of feed efficiency must be done with caution. It is important to remember that selection on a single trait may have unintended consequences on other important traits and their use should be a part of a selection index. New tools are being developed that offer the potential to select for more specific traits that are difficult to measure. An alternative approach has been the development of genomic markers that identify a region of DNA associated with a known region of DNA rather than the actual gene of interest. The approach is to create an animal population that has been intensely measured for production traits and then scan their DNA for DNA sequences that are unique to animals with specific traits. The sequence differences can create a set of DNA markers.